If you want to find even number of table rows then it's not a difficult task now, we can do this easily with the help of jQuery :even selector.

The :even selector will choose all those components which have even index i.e 0,2,4,6 etc.

Most of the time we have to use :even selector  together with another selector for choosing those  component which have even index .

In below example we are finding all table rows which have even index value with the help of :even selector and then making their background  grey.

Below is the html code for finding even number of table rows  :-

<table border="1">
<tr><td> Index value 0</td></tr>
<tr><td> Index value 1</td></tr>
<tr><td> Index value 2</td></tr>
<tr><td> Index value 3</td></tr>
<tr><td> Index value 4</td></tr>
<tr><td> Index value 5</td></tr>
<tr><td> Index value 6</td></tr>
</table>
<button id="btn">Click to see the effect</button> 

Below is the jQuery code :-

$('#btn').click(function(){ 
$( "tr:even" ).css( "background-color", "grey" );
});

Working demo:- https://jsfiddle.net/bqp9hje5/6/